The present invention relates to phenolic antioxidant compositions having improved characteristics as regards their resistance to coloration and their ability to stabilize organic polymers, such as rubbers and plastics materials in general. The invention also relates to the use of the said antioxidant compositions for stabilizing the said organic polymers.
The use of substances having stabilizing characteristics, particularly antioxidants, is known in the production and/or working of rubbers and plastics materials in general. In the preparation of these organic polymers, and during their conversion, degradation phenomena may occur as a result of the oxygen in the air, light and/or the working temperature, with consequent undesired variations in the rheological properties of the polymers themselves. In particular, oxidation may occur which, as a final result, brings about the breakage of the polymeric chain with consequent diminution in the viscosity, average molecular weight, flexibility or resilience of the polymer.
In order to avoid these disadvantages, it is usual to add particular substances which can act as antioxidant stabilizers either during the polymerization reaction which results in the formation of the polymer and/or to the polymer formed and/or during the working to which the polymer is subjected to convert it into manufactured articles. For this purpose phenols are particularly effective in which the phenolic hydroxyl group is sterically hindered by the presence of voluminous substituents on the ring. These phenols in fact have the capacity to retard the oxidation by acting as radical inhibitors.
The said phenolic antioxidants have an undesirable tendency to coloration, although to different degrees. In time they in fact tend to yellow or take on other colour tones and this is a disadvantage in that the colour is transmitted to the organic polymers which are intended to be stabilized. Thus, for example, in the case of the production of SBR rubbers (styrene-butadiene) and ABS rubbers (acrylonitrile, butadiene and styrene) the tendency to coloration of phenolic antioxidant emulsions which are added to the rubber latex before coagulation is particularly undesirable as is also the tendency of organic polymers in general to become coloured during their working at high temperatures.
In order to avoid these disadvantages it is usual in the art to add specific substances to the phenolic antioxidants or their emulsions which are able to improve the resistance of the antioxidants themselves to coloration. Thus, according to Japanese Pat. No. 7230897 of the Aug. 10, 1972, sodium formaldehyde sulphoxylate is used as a colour stabilizer for aqueous emulsions of phenolic antioxidants of the type 2,6-di-tertbutyl-4-methylphenol. According to U.S. Pat. No. 3,887,447, a hydrazine is used for this purpose in combination with phenolic antioxidants of the type 2,4-ditert-butyl-4-nonylphenol. Other substances used in the art are derivatives of hydroxylamine, such as the oximes and alkylhydroxylamines.
The use of said substances to confer resistance to coloration on the antioxidants does not always give satisfactory results. Indeed, they display their effect in rather high concentrations in the phenolic antioxidants or are not soluble in the said antioxidants, which limits their use, for example, solely to aqueous emulsions of the antioxidants. Finally, none of these added substances improves the stabilizing effect of the phenolic antioxidant on the organic polymers to which they are added.